Numerical modelling of a dual-rotor marine current turbine in a rectilinear tidal flow

TLDR: In this paper, numerical simulation is used to investigate a counter-rotating dual-rotor marine current turbine (MCT) that is aligned for a rectilinear tidal current, and results of power and thrust coefficients and the mean axial velocity in the wake are compared with that of the blade element momentum (BEM) method coupled with the Park wake model.

About: This article is published in Ocean Engineering.The article was published on 2020-03-15 and is currently open access. It has received 6 citations till now. The article focuses on the topics: Rotor (electric) & Turbulence kinetic energy.

Figures

Figure 3: Mesh details for the turbine

Figure 10: Schematic sectional wake region at axial distance X (a), and normalized mean axial velocity downstream of a single-rotor tip-pitched at θT = 2 ◦, TSR=5 (b)

Figure 13: Local streamline of the rear rotor of dual-rotor (X=4D) with TSRfront = 5 and variable TSRrear (TI=1%, RANS, U is the mean axial velocity downstream a single-rotor tip-pitched θT = 2 ◦ )

Figure 4: Pressure coefficient of blade profile at 0.9R, single-rotor, TSR=5 θT = 2 ◦

Figure 5: Isolated front rotor’s power (a) and thrust (b) coefficients variations with TSR, (E387 data is from [17, 26]) (TI=1% in RANS)

Figure 6: Isolated front rotor’s power (a) and thrust (b) coefficients at different turbulence intensity levels (NACA0012, θT = 2 ◦, RANS)

Frequently asked questions

Q1. What are the contributions in "Numerical modelling of a dual-rotor marine current turbine in a rectilinear tidal flow" ?

In this study, numerical simulation is used to investigate a counter-rotating dual-rotor marine current turbine ( MCT ) that is aligned for a rectilinear tidal current. 

Q2. What is the turbulence intensity of a single-rotor tip-pitched?

For a single-rotor tip-pitched at θT = −2◦, a high turbulence intensity region (such as TI > 0.28) extends to 5D downstream after the rotor and 0.6D in radial direction, while the high turbulence intensity region is mainly340 constrained near the hub for a single-rotor tip-pitched at θT = 2 ◦. 

Q3. What is the way to improve the performance of a rotor?

The unsteady RANS simulation can also be used to improve the prediction of performance and load on rear rotor which operates in turbulent windmill state and experiences370 larger fluctuations during one periodic cycle. 

Q4. How much flow separation is observed at high TSRU?

At low305 TSRU (2.91, 3.75), there are large flow separation from the rear surface of the blade, while no flow separation is observed at high TSRU , such as TSRU = 6. 

Q5. What is the effect of the LSB on the profile hydrodynamic efficiency?

The LSB is near the inner board of blade and thickens the boundary layer, thus contributes to the295 decrease of profile hydrodynamic efficiency, CL/CD. 

Q6. Why is the pitch control system more expensive and vulnerable than that of a wind turbine?

Due to higher waterproof standard, the electrical pitch system for tidal turbine is more expensive and vulnerable than that of a wind turbine which normally operates in a dry environment [2]. 

Q7. What is the power coefficient of the NACA0012?

The power coefficient of the E387 rear rotor was found to be highly negative, less than - 0.5 for TSR = 4 when using RANS or BEM, hence justifying again the use of the symmetric profile NACA0012 for180 the dual rotor configuration. 

Q8. What is the size of the vortex for a single-rotor tip-pitched?

The vortex size is about 0.5D for a single-rotor tip-pitched at θT = 2◦, while a much larger size of vortex (6D) is observed for a single-rotor tip-pitched at θT = −2◦. 

Q9. How is the axial mean velocity of the front rotor calculated?

Based on the CT of front rotor and area-averaged axial mean velocity from RANS results, the calibrated value of k is 0.017( TI=15%) using the least square fitting [36]. 

Q10. What is the optimum TSR of the NACA0012 rotor?

the optimum TSR of the NACA0012 rotor (θT = 2◦) is about 4.75, which is higher than that of the E387 turbine (TSR=4.25) [29]. 

Q11. What is the effect of the vortical wake on the rear rotor?

The vortical wake shedded by the front rotor is seen to reduce the wake behind the rear rotor compared to the single-rotor wake and thus has the potential actually to mildly increase the rear315 rotor hydrodynamic efficiency, explaining the high CP seen in Fig. 9 (b). 

Q12. What are the power coefficients of a dual-rotor turbine?

The power coefficients of front and rear rotor of a dual-rotor turbine are190 denoted as Cfrontp,dual and C rear p,dual, respectively. 

Q13. What is the wake shape of a single rotor?

For a single-rotor tip-pitched at θT = 2◦, the wake shape is axisymmetric, while a non-axisymmetric wake is observed for a single-rotor tip-pitched at θT = −2◦ and a dual-rotor with rotorspacing of 4D.330 Fig. 19 presents side view of the turbulence intensity of a single and dual-rotors operating at TSR 5 with ambient turbulence intensity of 1%. 

Q14. What was the maximum reduction in CP observed when the ambient turbulence intensity increased?

Mycek’s experimental work [27] showed that a maximum reduction of 13% in CP was observed when the ambient turbulence intensity increased from a low value of 3% to a high value of 15%. 

Q15. What is the maximum CP of a dual-rotor wind turbine?

For a wind farm, the numerical results from Vaselbehagh [7] showed that the dual-rotor turbines produced 22.6% more power than the single-rotor turbines. 

Q16. What is the turbulence intensity of a dual-rotor?

For a dual-rotor with X=4D, an interesting observation is that a turbulence intensity region is developed in front of rear rotor. 

Q17. What is the value of the empirical wake expansion rate?

The value of the empirical wake expansion rate, k, is an important parameter for the velocity deficit calculation in the Park model.